The present invention pertains to a portable engine, and, in particular, to a single cylinder internal combustion engine of the size and type adapted for use in power equipment such as that used in lawn and garden, general utility and snow removal operations. Such equipment includes but is not limited to lawnmowers, snow throwers, generators, string trimmers, leaf blowers, ice augers, earth movers, etc.
A variety of portable engines which are relatively lightweight have been employed with outdoor or lawn and garden power equipment such as lawnmowers, string trimmers and the like. While both four cycle and two cycle engine designs have previously been utilized, four cycle engines have generally emerged as the preferred design from the standpoint of reducing exhaust and noise emissions. In particular, recent legislation has reduced allowable exhaust emission levels to a point where the engine must be carefully designed to comply with promulgated emission levels, and four cycle engines typically burn cleaner than two cycle engines.
One shortcoming of some commercially available four cycle engines that undesirably leads to higher emissions relates to their propensity to distort in shape. As the engine heats up during usage, the thermal expansion of the engine cylinder block components may produce bore distortions which allow leakage, such as lubricating oil, to pass the piston rings and pollute the engine exhaust. In particular, due to weight and space restrictions inherent in the utilization of these portable engines, and in order to accommodate other mechanical workings of the engines such as drive components for an overhead camshaft, the cylinder bore wall thickness may vary markedly around the bore perimeter. In addition, the walls may be less rigid than optimal because a thin inner wall must be provided to separate multiple internal chambers. In addition, reinforcing ribbing may be withheld due to spacing requirements. These wall thickness variations and lack of rigidity may result in a non-uniform expansion or distorting of the cylinder bore during combustion pressure and thermal cycling, and consequently an unclean engine combustion may occur. A further consequence of such distortion producing leakage is to form oil-based deposits in the combustion chamber. It is well known that these deposits are an important source of the emission of volatile organic compounds, a critical constituent in the control of exhaust emissions. Build-up of these deposits over time is the main contributor to the deterioration of the control of exhaust emissions over the useful life of an engine.
Another potential source of cylinder bore distortion stems from the use of a separate head and cylinder. When a cylinder head is fastened to the cylinder block, the point loading around the cylinder bore which occurs with head bolt torquing may create sufficient bore distortion to compromise the seal with the piston. The head gasket normally introduced between the cylinder and head creates additional bore distortion concerns. For example, because the head gasket serves as a heat transfer barrier and thereby does not uniformly distribute the heat energy over the cooling surfaces of the engine, distortion potential of the cylinder bore associated with thermal expansion may be exacerbated.
Another shortcoming of some existing single cylinder engines relates to their lubrication system. Many engines depend on a continual splashing of the lubricant collected in the sump to lubricate the moving engine components. This splashing technique is not entirely satisfactory as it tends to be less reliable in thoroughness than pressurized lubrication. Further, because splash-type lubrication demands that the engine remain in a designed-for orientation to ensure the oil splashers extend into the collected lubricant, the orientations at which the engine can operate may be limited, thereby hindering engine applications. In other systems, a pump immersed in the lubricant collected in the crankcase sump distributes that lubricant around the engine. In addition to having a limited range of engine orientations at which a given pump will function, this configuration has several disadvantages. For example, a separate pump is required which may increase the engine weight, engine cost and be inconvenient to access for servicing. In addition, the amount of oil is limited by the crankcase volume. Still other engines which use a dry sump lubrication system require an additional pump mechanism to pump the sump contents to a reservoir, and this additional pump adds undesirable weight and cost.
The need for flywheels introduces other problems in portable engines. Due to space constraints, flywheels are typically mounted on the crankshaft at a position external of the engine housing and in a cantilevered fashion. To support this cantilevered flywheel mass without failure, the crankshaft must be formed with a stronger shaft than would be required without an external flywheel. Regardless of whether this stronger shaft is obtained by using a stronger material or by providing a larger diameter shaft, the overall weight of the engine is likely to be increased, and the ease of portability of the engine is thereby diminished. In addition, flywheels are frequently formed separately from the crankshaft and then rotatably fixed together via keying. Unfortunately, during aggressive or emergency stopping which can occur by accident or by use of braking devices, the inertia of the flywheel can lead to breakage of the key between the crankshaft and the flywheel, which renders the engine nonoperational.
Thus, it is desirable to provide a small internal combustion engine which overcomes these and other disadvantages of prior art engines.